1. Field of the Invention
This invention relates to a spark plug for an internal combustion engine suitable for a vehicle, a cogenerator, a gas transporting pump, or the like.
2. Description of the Related Art
A large engine, which is applied to a large generator using gaseous fuel (cogenerator), a gas transporting pump, or the like, usually includes a large spark plug having, for example, a housing outer diameter of 18 mm, an insulator head portion diameter of 14 mm, a central electrode diameter of 2.5 mm, and an entire length of 115 mm. This kind of large engine is always operated without intervals. Therefore, the large spark plug for the engine is especially required to have long life in order to improve a maintenance property and in order to reduce plug exchanging cost.
As shown in FIG. 1, this kind of spark plug 90 typically has an insulator 4 and a resistor 69 disposed in a through hole 41 of the insulator 4. In the through hole 41, a central electrode 2, a low melting point glass 70, and a conductive sintered member 71 are disposed on a side of the resistor 69, and a terminal electrode 21 and a spring 23 fixing the resistor 69 are disposed on the other side of the resistor 69. A housing 5 holds the insulator 4, and an earth electrode 3 and a screw portion 52 for attachment are provided on the housing 5. The central electrode 2 and the earth electrode 3 define a spark gap G therebetween. The resistor 69 is for lowering radio noise caused by spark discharge, and disposed adjacently to the spark gap G to sufficiently prevent the radio noise. The resistor 69 is formed by sintering and pressing resistive powders as a solid resistor, or by welding resistive material including glass powders such as borosilicate system glass, ceramic powders such as zirconia, and carbon system resistive material such as carbon black in a high temperature furnace and forming into a columnar shape.
The spark gap G needs to be prevented from being enlarged and required voltage necessary for causing spark discharge needs to be prevented from being increased, in order to secure long life of the spark plug 90. Because of this, a noble metal cap 28 made of noble metal or an alloy of the noble metal is used as a discharge member. Recently, however, the large gas engine has been required to improve combustion efficiency and to be operated under high compression and a lean-burn state so that fuel consumption is improved and so that an amount of exhaust gas is decreased. Accordingly, the spark plug 90 is forced to be operated under hard conditions such as increases in required voltage, thermal load, ignition energy, and the like.
When the spark plug 90 is operated under such hard conditions, the resistor 69 disposed adjacently to the spark gap G is exposed to high temperature for a long time period to have increased resistance. The increased resistance of the resistor 69 may cause ignition failure of the engine. Further, sealing property can easily deteriorate due to high pressure gas passing through an interval between the central electrode 2 and the through hole 41.